1. Field of the Invention
The present invention is directed to a marker for use in a magnetic anti-theft security system, the marker being of the type which is composed of an oblong alarm strip composed of an amorphous ferromagnetic alloy, and at least one activation strip composed of a semi-hard magnetic alloy.
2. Description of the Prior Art
Magnetic anti-theft security systems and markers therefor of the above type are well known and are described in detail in, for example, European Application 0 121 649, and PCT Application 90/03652. First, there are magneto-elastic systems wherein the activation strip serves for activation of the alarm strip by magnetizing it; second, there are harmonic systems wherein the activation strip, after being magnetized, serves for the deactivation of the alarm strip.
The alloys with semi-hard magnetic properties that are employed for the pre-magnetization strip include Coxe2x80x94Fexe2x80x94V alloys, which are known as VICALLOY, Coxe2x80x94Fexe2x80x94Ni alloys, which are known as VACOZET, as well as Fexe2x80x94Coxe2x80x94Cr alloys. These known semi-hard magnetic alloys contain high cobalt parts, some at least 45 weight %, and are correspondingly expensive.
In their magnetically finally annealed condition, further, these alloys are brittle, so that they do not exhibit adequate ductility in order to adequately meet the demands given display elements for anti-theft security systems. One important demand, namely, is that these activation strips should be insensitive to bending or deformation.
In the meantime, further, a switch has been made to introducing the markers in anti-theft security systems directly into the product to be secured (source tagging) Such source tagging imposes the additional demand that the semi-hard magnetic alloys should be able to be magnetized from a greater distance or with smaller fields. To satisfy this additional demand, it has been shown that the coercive force Hc must be limited to values of at most 24 A/cm.
On the other hand, however, an adequate opposing field stability is also required which determines the lower limit value of the coercive force. Only coercive forces of at least 10 A/cm are thereby suited.
Further, the remanence should be optimally slight under bending or tensile stress. A change of less than 20% is prescribed as guideline.
It is an object of the present invention to provide a marker of the above-described type for a magnetic anti-theft system, having an activation strip which satisfies the above demands for source tagging.
This object is inventively achieved in a marker having an activation strip composed of a semi-hard magnetic alloy comprising 8 to 25 weight % nickel, 1.5 to 4.5 weight % aluminum, 0.5 to 3 weight % titanium and the balance iron.
The alloy can further contain 0 to 5 weight % cobalt and/or 0 to 3 weight % molybdenum or chromium and/or at least one of the elements Zr, Hf, V, Nb, Ta, W, Mn, Si in individual parts of less than 0.5 weight % of the alloy and in an overall part of less than 1 weight % of the alloy and/or at least one of the elements C, N, S, P, B, H, O in individual parts of less than 0.2 weight % of the alloy and in an overall part of less than 1 weight % of the alloy.
The alloy is characterized by a coercive strength Hc of 10 to 24 A/cm and a remanence Br of at least 1.3 T (13,000 Gauss).
The inventive alloys are highly ductile and can be excellently cold-worked before the annealing, so that crossectional reductions of more than 90% are also possible. An activation strip having a thickness of less than 0.05 mm can be manufactured from such alloys, particularly by cold rolling. Further, the inventive alloys are characterized by excellent magnetic properties and resistance to corrosion.
A preferred alloy is a semi-hard magnetic iron alloy according to the present invention that contains 13.0 to 17.0 weight % nickel, 1.8 to 2.8 weight % aluminum as well as 0.5 to 1.5 weight % titanium. By reducing the aluminum content, the magnetostriction can, in particular, be especially favorably set.
Typically, the activation strips are manufactured by melting the alloy under vacuum and casting to form an ingot. Subsequently, the ingot is hot-rolled into a tape at temperatures above 800xc2x0 C., then intermediately annealed at a temperature above 800xc2x0 C. and then rapidly cooled. A cold working, expediently cold rolling corresponding to a crossectional reduction of approximately 90% is followed by an intermediate annealing at approximately 700xc2x0 C. A cold working, expediently cold rolling corresponding to a crossectional reduction of at least 60%, preferably 75% or more subsequently occurs. As last step, the cold-rolled tape is annealed at temperatures from approximately 400xc2x0 C. to 600xc2x0. The activation strips are then cut to length.